Are all low molecular weight heparins equivalent in the management of venous thromboembolism?

Low molecular weight heparins are replacing unfractionated heparin in a number of clinical indications because of their improved subcutaneous bioavailability and more predictable antithrombotic response. Clinical trials have demonstrated that low molecular weight heparins are at least as safe and effective as unfractionated heparin for the initial treatment of venous thromboembolism, and unfractionated heparin and warfarin for primary and secondary thromboprophylaxis. The mechanism behind the antithrombotic action of low molecular weight heparins is not fully understood but is likely to involve inhibition of coagulation factors Xa and IIa (thrombin), release of tissue-factor-pathway inhibitor, and inhibition of thrombin activatable fibrinolytic inhibitor. Different low molecular weight heparins have been shown to have various effects on coagulation parameters. Seven low molecular weight heparins are currently marketed worldwide, each demonstrated distinct chemical entities with unique pharmacokinetic and pharmacodynamic profiles. Each low molecular weight heparin is approved for specific indications based on the available efficacy and safety data for that product. The relative efficacy and safety of the low molecular weight heparins are unclear because there have been very few direct comparisons in randomized clinical trials. While recommending low molecular weight heparins for the prevention and treatment of venous thromboembolism, clinical guidelines have not specified individual agents. National and international organizations recognize that low molecular weight heparins are distinct entities and that they should not be used interchangeably in clinical practice. Each low molecular weight heparin should be used at the recommended dose when efficacy and safety data exist for the condition being treated. When these data are not available, the dosing and administration of low molecular weight heparins must be adapted from existing data and recommendations.

Anticoagulation therapy

Despite refinements and standardization in the use of anticoagulants, many problems remain for clinicians. Dr. Crowther describes appropriate starting and maintenance doses of warfarin, factors accounting for inter- and intra-observer variability and importantly, the management of the over-anticoagulated patients and bleeding patients. Dr. White compares unfractionated heparin (UFH) and low molecular weight heparin (LMWH) and addresses whether there truly are differences in the efficacy and safety of different LMWH's for both arterial and venous indications. Dr. Ortel discusses the management of the problem patient who requires anticoagulants, the management of heparin-induced thrombocytopenia, the pregnant patient, the obese patient, patients who have renal insufficiency and/or liver disease, patients with malignant disease, and other challenging patient populations.

Stasis ulcers refractory to therapy--accelerated healing by treatment with clopidogrel +/- dalteparin: a preliminary report

Stasis ulcers are commonly the result of chronic venous insufficiency. We have recently assessed 15 patients with stasis ulcers that failed to heal after one year of routine wound care. All patients demonstrated a defect in hemostasis, and a biopsy revealed livido vasculitis. Eleven of fifteen patients were treated with clopidogrel and dalteparin, and 4 of 15 patients were treated with clopidogrel alone. Thirteen of fifteen patients (86.6%) completely healed within three months of starting antithrombotic therapy. Patients with stasis ulcers and chronic venous insufficiency who fail to heal with routine wound care should be subjected to biopsy, a procoagulant defect evaluation, and initiation of clopidogrel and dalteparin therapy if a defect is found.

Low molecular weight heparins: differences and similarities in approved preparations in the United States

There is adequate preclinical data to support the differential biochemical and pharmacological behavior of the currently approved low molecular weight heparins (LMWHs) in the United States. Initial studies on the anti-Xa, anti-IIa, and U.S. Pharmacopoeial (USP) potencies have clearly demonstrated differences among these products. Furthermore, the ratios between the anti-X and anti-IIa activities vary from one product to another. This is primarily due to the composition of each product manufactured by using different patented methods. Studies in pharmacologic animal models, using gravimetric dosages or adjusted anti-Xa dosages of the LMWHs, produce product-specific results. The pharmacokinetics and pharmacodynamics of each product also vary markedly and are not predictable on the basis of any pharmacopoeial potency designation. These agents are capable of releasing tissue factor pathway inhibitor (TFPI), an inhibitor of the coagulation process. Its release is also dependent on the type of LMWH. In the United States enoxaprin, dalteparin, and ardeparin have been approved for DVT prophylaxis. Only enoxaparin and dalteparin have been approved for the acute coronary syndrome. Recently the clinical differentiation among these LMWHs has been demonstrated in the treatment of acute coronary syndrome. Similarly, when these drugs are used at high dosages, they are expected to produce product-specific pharmacodynamic effects. It must be noted that while these drugs may be interchangeable at clinically optimized/approved dosages, these drugs are not interchangeable at equivalent anti-Xa dosages. Even at optimized dosages, the clinical provile of each drug may be different. Thus, each of the LMWHs should be considered a distinct entity and their use in a given clinical situation should be validated in proper clinical trials.